D sync generator

The basic frequency of the sync generator is a 12.096MHz clock, generated by a crystal-controlled oscillator consisting of crystal Yl. inverter R3 and several descrcte components. The output of the oscillator is 12MHz. Examination of this signal with an oscilloscope (at pin 4 of R3) should show a period (or one cycle) of approximately 83 nanoseconds (.083 microseconds).

This base frequency is then divided down by binary counters R4, P4, and N4. These provide the various horizontal synchronization frequencies (1H thru 256H). The final output of this horizontal counter chain is 256H (at pin 13 of N4). This signal is, in effect, a division of the base frequency by 768, or 15,750 Hz. The period of 256H is about 63.5 microseconds (15,750 Hz). The 256H signal, as well as other horizontal signals, are used to generate the H BLANK and H SYNC timing pulses at flip-flop P5. See Figure 4-9, Horizontal Sync Timing Diagram for the relative timing of these waveforms.

The H SYNC signal is used to clock two stages of vertical sync counters P6 and N6. The H SYNC signal is divided down by these counters to form the various vertical syn-chrozintation frequencies (IV thru 128V). These two stages are used, in effect, to do a divide by 262 of the H SYNC signal. This results in the final output of these counters (128V at pin 11 of N6) of 60 Hz (the period is approximate ly 16.6 milliseconds). The various vertical frequencies address a sync PROM (N5) whose data is latched at the output of P7. The outputs of this latch are the various polarities and phases of the vertical timing frequencies used to synchronize the TV monitor display. See Figure 4-9, Vertical Sync Timing Diagram, for the relative timing of these waveforms.

The end result of the horizontal and vertical timing waveforms is to synchronize the TV monitor display. This display consists of 262 horizontal lines per frame, with each line being the time equivalent of 768 (12MHz) clock pulses. Each frame is repeated 60 times per second providing the necessary frequency of display refresh to provide a stable, non-flickering display.

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